1. Field of the Invention
The present invention relates to a semiconductor processing apparatus and a semiconductor processing method which irradiate energy beams onto a semiconductor film provided on a substrate to crystallize the semiconductor film by a heat treatment.
2. Description of the Related Art
Flat type display devices, such as a liquid crystal display device and an organic EL (Electro Luminescence) display device, use thin film transistors (Thin Film Transistors: TFTs) as switching elements for effecting active matrix display of a plurality of pixels.
Thin film transistors include a polycrystalline silicon TFT which uses a polysilicon (poly-Si), microcrystalline silicon (μc-Si) or the like in an active region, and an amorphous silicon TFT which uses an amorphous silicon (amorphous Si) in an active region. Of the two types of TFTs, the polysilicon TFT has a feature that the carrier mobility is greater by ten to 100 or so times as compared with the amorphous silicon TFT, and thus has an excellent characteristic as a material for a switching element.
Because existing pixel switching elements for driving a liquid crystal do not need such a high carrier mobility, amorphous silicon TFTs are generally used as pixel switches. It is however desirable to use switching elements which have a higher carrier mobility than that of the amorphous silicon TFT in organic EL or high frame usage which is expected to be used in the next generation TFTs.
However, the characteristics of the polycrysilicon TFT, such as carrier mobility and threshold voltage, are likely to vary from one device to another due to the presence of both a crystal region and a grain boundary region in a channel region which is the core of the transistor. It is said that the carrier mobility and the device variation have a trade-off relation such that as the carrier mobility becomes greater, the device variation becomes larger.
Multiple studies have been done on such a relation originating from the unique grain boundary of the polysilicon TFT.
For example, JP-A-2004-87535 (Patent Document 1) discloses a technique of irradiating an excimer laser beam over a diffraction grating mask in addition to a control method for the crystal orientation in repetitive irradiation. That is, positional control on the grain boundary is carried out with the diffraction grating mask in such a way that crystal growth occurs at a point of the intensity-modulated minimum light intensity as the origin, and the position of the grain boundary is set to a point of the maximum light intensity.
As another technique, a scheme of crystallization with laser scanning is disclosed. For example, JP-A-2004-87667 (Patent Document 2) discloses SELAX (Selectively Enlarging Laser Crystallization). This method moves (scans) a continuous wave (CW) laser beam or a pseudo CW laser beam with a very high pulse frequency of several tens of MHz or so in one direction in relative to a silicon film deposited on a substrate.
The laser scanning causes a crystal to grow in one direction. The grain boundary is formed substantially in parallel to the direction of growth. Therefore, the electric conduction in the direction of crystal growth reduces the density of the grain boundary where carriers cross, thus making it possible to increase the carrier mobility.
The closer to a monocrystal the channel portion becomes, the more prominent a difference in in-plane crystal orientation, a slight angular difference in plane orientation, and a difference in internal defect become. It is therefore desirable to form a crystal film of a higher quality of the level of a monocrystalline Si substrate to suppress a device variation.
JP-A-2007-281422 (Patent Document 3) discloses a technique of forming a polycrystal with a controlled number of grain boundaries where carriers cross, not making silicon in the channel into a monocrystal. Further, JP-A-2007-281420 (Patent Document 4) discloses a technique of scanning a channel region with a spot laser beam at equal pitches to form grain boundaries. This makes it possible to form a transistor with a smaller device variation while having a relatively high carrier mobility.